Reconfigurable and Integrable Optical Logic Gate Based on a Single Soa

ABSTRACT

An optical logic gate ( 10 ) comprising inputs ( 12 ) for optical signals on which to perform a chosen logical operation. An SOA ( 11 ) element receives such input signals to be piloted thereby in saturation and its output is connected to at least one optical filter ( 14, 15, 16 ) that filters components of signals output from the SOA and which represent a desired logical result of the signals input at the gate so that at the output ( 13 ) of the filter there is an optical signal as the result of the desired logical operation. A probe signal ( 17 ) can also be provided. An appropriate combination of power of the input, power and probe signal wavelength and central wavelength of the filter allows obtaining a plurality of logic functions such as NOR, NOT, inverted XOR, AND, OR.

This invention relates to an integrable diagram based on a single SOA torealize ultrafast and reconfigurable optical logic gates able to producefor example NOT, AND, OR, NOR and inverted XOR functions.

The development of all-optical technologies is fundamental for realizingfuture telecommunications networks where all the node functionalitieswill have to be carried out in the optical domain.

All-optical functions for add-drop multiplexing, packet synchronization,clock recovery, address recognition, signal regeneration et cetera areessential to avoid optoelectric conversions, which can represent thebottleneck to obtaining broadband and flexible networks.

All-optical logic gates are a key element in the realization of suchfunctionalities. In the literature, there have been proposed someall-optical logic gate diagrams using non-linear effects in the opticalfibers or in semiconductor devices. In general, that already proposeddoes not offer satisfactory performance or integration possibilities.

The general purpose of this invention is to remedy the above mentionedshortcomings by making available optical logic gates that would bereconfigurable and integrable based on a single SOA.

In view of this purpose it was sought to provide in accordance with thisinvention an optical logic gate comprising inputs for optical signals onwhich to perform a chosen logical operation, an SOA element thatreceives such input signals to be piloted thereby in saturation andwhose output is connected to at least one optical filter that filterssignal components output from the SOA and that represent a desiredlogical result of the signals input at the gate so that at the output ofthe filter there is an optical signal as the result of the desiredlogical operation.

To clarify the explanation of the innovative principles of thisinvention and its advantages compared with the prior art there isdescribed below with the aid of the annexed drawings a possibleembodiment thereof by way of non-limiting example applying saidprinciples. In the drawings:

FIG. 1 shows a diagram of a reconfigurable logic gate realized inaccordance with the principles of this invention,

FIG. 2 shows an experimental preparation for the test of a devicerealized in accordance with this invention,

FIG. 3 shows (left) a graph of sequences of input signals and thecorresponding logical output (right) and the corresponding roughdiagrams for different types of logic gate obtained with a device inaccordance with this invention, and

FIG. 4 shows the BER of the back-to-back input signals and of thelogical output in the various configurations.

With reference to the figures, FIG. 1 shows a diagram of areconfigurable logic gate designated as a whole by reference number 10and based on a known Semiconductor Optical Amplifier (SOA) 11.

The gate 10 comprises inputs 12 and one or more logical outputs 13connected to the output of the SOA by means of known all opticalPass-Band Filters (PBF) 14, 15, 16.

The signals on which to perform the logical operation are input to theSOA through an input element made up of at least one known opticalcoupler 20.

As set forth below, with the diagram proposed, logical functions NOT,AND, OR, NOR and inverted XOR can be easily realized. This is doneutilizing simultaneously or alternatively Four Wave Mixing (FWM) andCross Gain Modulation (XGM) or Cross-Phase Modulation (XPM) in thesingle SOA.

The use of an SOA was found particularly advantageous for therealization of different logic functions since SOAs can exhibit a strongexchange of the refraction index together with high gain. In addition,differently from the fiber devices, SOAs allow photon integration.

To clarify structure, sizing and functioning of the device in accordancewith this invention the various logic functions obtainable are describedbelow.

In the diagram of FIG. 1, A and B indicate the signals that must beprocessed and whose wavelengths are respectively λ_(A) and λ_(B).

The inverted XOR function is described first. This logic function isobtained by simultaneously using the FWM between the two A and B signalsaligned in polarization and the XGM on a co-propagating probe signal(produced by an appropriate source 17) and whose λ_(probe) wavelength isthe same as one of the FWM terms generated (λ_(probe)=λ_(FWM)). Theprobe signal is always input into the SOA through the input elementwhich advantageously comprises a second optical coupler 21 downstream ofthe first.

The diagram proposed is capable of processing either NRZ (NonReturn-to-Zero) or RZ (Return-to-Zero) signals. In the former case, theprobe is a Continuous Wave (CW) light while in the latter case it is apulsating clock.

In order to avoid phase interference between probe and FWM component,the probe channel is launched in the SOA with polarization orthogonal tothe signals and consequently to the FWM term.

Each A, B signal has peak input power P_(A), P_(B) corresponding to thehigh logic level, which is high enough to saturate the device and inducea high efficiency FWM effect (that is to say P_(A), P_(B)>=P_(satSOA)).The peak power of the probe is instead chosen low enough to avoidsaturation of the SOA (that is to say P_(probe)<P_(satSOA)). A pass-bandfilter (15) centered on λ_(FWM) supplies the output signal for theinverted XOR logic gate.

In this manner, when both the A and B signals are present (case 11) thepower at input is such that the FWM component is generated andsimultaneously the probe channel 17 experiences very low gain in thesaturated device 11.

After the SOA, the term FWM is present and traverses the filter 15 sothat the output of the logic gate is at high level.

Contrariwise, if both the A and B signals are absent (case 00) the FWMeffect is not present and the SOA is not saturated. Therefore the probesignal experiences a strong amplification and at the output of thefilter there will be a high power level. By means of an appropriatesetting of the probe channel input power it is possible to equalize thehigh power level at the output of the inverted XOR gate in the twocases, 00 and 11.

If on the other had only one of the two A or B signals is there (cases10 and 01), the FWM is not there but the SOA is saturated in any case(the input power of a single channel is sufficient to saturate thedevice), severely reducing the probe signal gain. Therefore, in cases 10and 01, at the centered optical filter output on λ_(probe)=λ_(FWM) thepower level is low.

Advantageously, for the purpose of avoiding signal distortions dependingon the pattern, a high powered counter-propagating CW pump 18 islaunched in the SOA, decreasing the mean life of the carriers andmaintaining the optimal saturation level in the SOA.

The same diagram can be used to obtain different logic gates whilekeeping the same input condition for the A and B signals.

In particular, if the probe channel is extinguished, the optical filteroutput centered on λ_(FWM) represents an AND logic function based on theFWM.

Changing the wavelength of the probe channel 17 so that it isλ_(probe)≠λ_(FWM) and using an optical pass-band filter 14 centered on aλ_(probe), the NOR signal is extracted. In this case, the NOR gate isbased on the XGM in the SOA.

But the NOT function can be obtained considering only an input signal inthe inverted XOR or NOR realizations while exploiting the XGM on theprobe channel.

Lastly, the OR function can be obtained by exploiting the XPM. In thiscase, the wavelength of the probe signal 17 is fixed to obtainλ_(probe)≠λ_(FWM). As long as the input power of each signal issufficient for saturating the device, the XPM effect induced by thepresence of a single signal or of both the signals causes a similarΔλ_(XPM) shift. Therefore if both the signals (case 11) or only onesignal (cases 01 or 10) are in the SOA, an optical filter 16 centered onλ_(filter)=λ_(probe)+Δ_(XPM) and with an appropriate band width canextract the probe signal.

If both signals are absent (00 case), the probe signal spectrum does notmove and the probe signal 17 goes out of the filter band 16. If theprobe input power is high enough to stay over the XGM effects, a highoutput will be obtained at the output of the filter in cases 11, 10 and01 and a low output in case 00, thus reproducing the OR logic function.

In the following table, the necessary conditions that must be respectedfor the P_(probe) power of the probe signal are summarized for thevarious logic functions, the wavelength λ_(probe) of the probe signaland the central wavelength λ_(BPF) Of the filter. The saturation powerP_(satSOA) of the SOA must always be less than the power P_(H) of theinput signals that must be considered the high logic level.

XOR\ AND NOR OR P_(probe) 0 < P_(probe) < 0 0 < P_(probe) < 0 <P_(probe) < P_(satSOA) P_(satSOA) P_(satSOA) λ_(probe) λ_(FWM) λ_(FWM)≠λ_(FWM) ≠λ_(FWM) λ_(BPF) λ_(FWM) λ_(FWM) λ_(probe) λ_(probe) + Δλ_(XPM)

The NOT function is the same as the inverted XOR or NOR function with asingle input (with the other zeroed or eliminated).

It is seen how the simple and integrable diagram shown in FIG. 1,including three different filters 14, 15, 16 at the output of the SOA 11(or alternatively a single known tunable filter 19) can be easilyreconfigured to obtain different logic gates just by checking thewavelength λ_(probe) and the input power P_(probe) of the probe signal17 or extinguishing it.

The Bit Error Rate (BER) measurements in case of 20 ps signals at 10Gbit/s confirm the high performance of the innovative reconfigurablediagram and its adaptability to long cascaded configurations.

Some experimental tests were made to verify the effectiveness of thedevice in accordance with this invention. Only the results with the NOT,AND, NOR and inverted XOR logic functions are shown. But similar resultsare also believed confirmed for the OR gate anyway.

FIG. 2 shows an experimental preparation used for the tests. To produceA and B signals and the probe signal, a known pulsed fiber active 10 GHzMode Locking (ML) laser and a supercontinuum generation was used, aseasily imaginable to one skilled in the art. Naturally, other sourcescan be used.

In particular, the A and B signals pulsed at 20 ps and the probe signalpulsed at 20 ps were obtained from a super continuum in 500 meters ofHighly NonLinear Fiber (HNLF) while filtering on appropriate BPF filtersat λ_(A)=1550.9 nm, λ_(B)=1552.5 nm, and λ_(probe)=λ_(FWM)=1549.3 nm orλ_(probe)≠λ_(FWM)=1546.1 mm. The wavelength of the counter propagatingpump CW was set at 1544 nm.

The semiconductor device used is a commercial SOA independent of thepolarization with signal gain of 31 dB to 1547 nm. Mean input power was3 dB, −15 dB and 10 dB respectively for signals, probe and pump.

To demonstrate the effectiveness of the diagram proposed, particular bitrates were considered for the A and B signals at the input of thereconfigurable logic gate of FIG. 1. The rates were obtained withappropriate modulation of the signal output by the generator. For oneskilled in the art this is clear from the diagram of FIG. 2.

In FIG. 3 on the left the input rates and the corresponding output ratesare shown by using the diagram respectively as inverted XOR, AND, NORand NOT. In FIG. 3 on the right are shown the rough diagrams for eachlogic gate implemented. It is seen how the various logic gates arecorrectly implemented.

FIG. 4 shows the BER curves obtained at the output of each logic gate byusing the same input rate. It can be seen that the penalty introduced at10⁻⁹ is 0.5 dB less than the worst input signal, thus adapting theproposed diagram even for long cascaded configurations. In addition, theAND logic gate has regenerative characteristics because of thesaturation effect of the SOA that compresses the high level noise.

It is now clear that the preset purposes have been achieved. Thereconfigurable and integrable all optical device based on XGM, FWM andXPM in a single SOA can be reconfigured easily to produce NOT, AND, OR,NOR and inverted XOR logic functions. BER measurements using 20 pspulsed signals at 10 Gbit/s have shown a penalty of less than 0.5 db foreach logic function considered to show the effectiveness of the diagrameven for cascaded configurations.

Naturally the above description of an embodiment applying the innovativeprinciples of this invention is given by way of non-limiting example ofsaid principles within the scope of the exclusive right claimed here.

1-12. (canceled)
 13. An optical logic gate comprising: inputs configuredto receive input optical signals on which a selected logical operationwill be performed; a semiconductor optical amplifier (SOA) elementoperative to receive the input optical signals and capable of operatingat saturation in response to the input optical signals; at least oneoptical filter connected to an output of the SOA element and operativeto filter components of optical signals output from the SOA element, theSOA output optical signal components representing a desired logicalfunction of the input optical signals, such that the filter is operativeto output an optical signal resulting from the desired logicaloperation.
 14. The optical logic gate of claim 13 wherein logicalfunctions of the SOA output optical signal components are generated byselectively utilizing one or both of Four Wave Mixing (FWM) and CrossGain Modulation (XGM), and Cross-Phase Modulation (XPM).
 15. The opticallogic gate of claim 13 wherein the SOA element has an input saturationpower value P_(satSOA) that is less than or equal to a power valuecorresponding to a high logic level for the input optical signals, suchthat upon an input optical signal assuming the high logic level, the SOAelement operates at saturation to induce a corresponding FWM effect. 16.The optical logic gate of claim 15 further comprising a probe signalsource connected to an input of the SOA element and operative toselectively generate a probe optical signal having a peak power valueP_(probe) that is less than the saturation power value P_(satSOA) of theSOA element.
 17. The optical logic gate of claim 16 wherein: if theoptical input signals comprise Non Return to Zero signals, the probeoptical signal comprises a Continuous Wave light; and if the opticalinput signals comprise Return-to-Zero signals, the probe optical signalcomprises a pulsating clock.
 18. The optical logic gate of claim 16wherein the logical operation performed on the input optical signals bythe optical logic gate is selected at least in part by varying a powerP_(probe) and a wavelength λ_(probe) of the probe optical signal. 19.The optical logic gate of claim 18 wherein the logical operationperformed on the input optical signals by the optical logic gate isselected by varying P_(probe) and λ_(probe) of the probe optical signalaccording to the following table: XOR\ AND NOR OR P_(probe) 0 <P_(probe) < 0 0 < P_(probe) < 0 < P_(probe) < P_(satSOA) P_(satSOA)P_(satSOA) λ_(probe) λ_(FWM) λ_(FWM) ≠λ_(FWM) ≠λ_(FWM)

where λ_(FWM) is equal to the wavelength of an FWM signal componentproduced by a Four Wave Mixing effect of the optical signals in the SOAelement.
 20. The optical logic gate of claim 18 wherein a NOT functionof one of the input optical signals is obtained by configuring theoptical logic gate for an inverted XOR or inverted NOR function.
 21. Theoptical logic gate of claim 19 wherein the optical filter has a centralwavelength λ_(BPF) comprising one of λ_(FWM), λ_(probe), orλ_(probe)+Δλ_(XPM) where Δλ_(XPM) is the deviation on the signalproduced by an Cross Gain Modulation effect in the SOA element, wherebythe selection of λ_(BPF) determines the logical operation to beperformed.
 22. The optical logic gate of claim 19 wherein the opticalfilter comprises a plurality of band pass optical filters, each bandpass optical filter having a central wavelength λ_(BPF) comprising oneof λ_(FWM), λ_(probe), or λ_(probe)+Δλ_(XPM), where Δλ_(XPM) is thedeviation on the signal produced by an Cross Gain Modulation effect inthe SOA element, whereby each band pass optical filter performs adifferent logical operation according to the value of its λ_(BPF). 23.The optical logic gate of claim 19 wherein the optical filter comprisesa filter having a central wavelength λ_(BPF) that can be selectivelyadjusted to be λ_(FWM), λ_(probe), or λ_(probe)+Δλ_(XPM), where Δλ_(XPM)is the deviation on the signal produced by an Cross Gain Modulationeffect in the SOA element, to perform one or more respective logicaloperations.
 24. The optical logic gate of claim 16 wherein the probesignal source is configured to generate the probe optical signal havinga polarization orthogonal to that of the input optical signals, andhence to the Four Wave Mixing (FWM) signal components, to avoid phaseinterference between probe optical signal and the FWM components. 25.The optical logic gate of claim 16 further comprising a signal sourceoperative to generate a counter-propagating pump optical signal input tothe SOA element to decrease a mean life of signal carriers and tomaintain an optimal saturation level in the SOA element to avoid patterndependent signal distortions.